Applicant's prior U.S. Pat. No. 3,374,027, shows trailer constructions having a base with which the base of the present construction may be identical. Such base construction is formed of hollow vertical and horizontal closed structural beams so arranged that the external surfaces of the respective sides, ends and bottoms thereof lie in common planes, and an aluminum or similar skin covers the ends, sides and bottom of the base so that it is buoyant, whereby the trailer may be used as a house boat. Such prior construction show the idea of adjustable side walls having posts connected at their upper ends by roof supporting elements in the form of rods turning downwardly at their ends to engage in the posts and movable upwardly to be removed from the posts for the downward and inward swinging thereof to collapse the body of the trailer.